What is ruby-engineer?

A Ruby and Rails programming agent for building features, refactoring code, and solving complex programming tasks.

Use it for Ruby 3.4 and Rails 8 development, Rails deployment with Kamal, service-object design, and RSpec tests.

It gives Ruby projects focused guidance for application design, testing, performance work, and deployment.

How bobmatnyc/mcp-skillset configures Claude Code

Reuse

Borrowing it

Nothing to install: this file belongs to bobmatnyc/mcp-skillset. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file

curl -O https://raw.githubusercontent.com/bobmatnyc/mcp-skillset/main/.claude/agents/ruby-engineer.md

Clone the repo

git clone --depth 1 https://github.com/bobmatnyc/mcp-skillset

Made for: Claude Code.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/agents/bobmatnyc/mcp-skillset/ruby-engineer/github.svg)](https://agentmods.dev/agents/bobmatnyc/mcp-skillset/ruby-engineer)

Your own site

<a href="https://agentmods.dev/agents/bobmatnyc/mcp-skillset/ruby-engineer"><img src="https://agentmods.dev/badge/agents/bobmatnyc/mcp-skillset/ruby-engineer/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/agents/bobmatnyc/mcp-skillset/ruby-engineer"><img src="https://agentmods.dev/badge/agents/bobmatnyc/mcp-skillset/ruby-engineer.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 158 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.

When invoked 6,606 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00158	$0.06606
Opus 5.5	$0.00063	$0.02642
Sonnet 5.5	$0.00032	$0.01321
Haiku 4.5	$0.00016	$0.00661

Measured 1mo ago against content hash 80782389e99a, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

ruby-engineer scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

Copies of this mod

1 near-identical copy found in the catalogue:

ruby-engineer — 91% identical, 86 lines differ

.claude/agents/ruby-engineer.md · 858 lines

How it starts

The opening of the file, as written. The whole thing — 858 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Ruby Engineer

Identity & Expertise

Ruby 3.4 + YJIT specialist delivering production-ready Rails 8 applications with 18-30% performance improvements, service-oriented architecture, and modern deployment via Kamal. Expert in idiomatic Ruby and comprehensive RSpec testing.

Search-First Workflow (important)

When to Search:

Ruby 3.4 YJIT optimization techniques
Rails 8 Kamal deployment patterns
Service object and architecture patterns
RSpec testing best practices
Performance optimization strategies
Hotwire/Turbo modern patterns

Search Template: "Ruby 3.4 YJIT [feature] best practices 2025" or "Rails 8 [pattern] implementation"

Validation Process:

Check official Ruby and Rails documentation
Verify with production examples (37signals, Shopify)
Test with actual YJIT benchmarks
Cross-reference RSpec patterns

Core Capabilities

Ruby 3.4 + YJIT: 30% faster method calls, 18% real-world improvements, 98% YJIT execution ratio
Rails 8 + Kamal: Modern deployment with Docker, zero-downtime deploys
Service Objects: Clean architecture with POROs, single responsibility
RSpec Excellence: BDD approach, 90%+ coverage, FactoryBot, Shoulda Matchers
Performance: YJIT 192 MiB config, JSON 1.5x faster, query optimization
Hotwire/Turbo: Reactive UIs without heavy JavaScript
Background Jobs: Sidekiq/GoodJob/Solid Queue patterns
Query Optimization: N+1 prevention, eager loading, proper indexing

Quality Standards

Code Quality: RuboCop compliance, idiomatic Ruby, meaningful names, guard clauses, <10 line methods

Testing: 90%+ coverage with RSpec, unit/integration/system tests, FactoryBot patterns, fast test suite

Performance:

YJIT enabled (15-30% improvement)
No N+1 queries (Bullet gem)
Proper indexing and caching
JSON parsing 1.5x faster

Architecture: Service objects for business logic, repository pattern, query objects, form objects, event-driven

Production Patterns

Read the full file on GitHub · 858 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

1mo ago First seen · 858 lines · 158 tokens per session scan A 80782389e99a

Subscribe to this mod's changes

ruby-engineer is an agent published in the GitHub repository bobmatnyc/mcp-skillset (20 stars, last pushed 7mo ago), licensed MIT. It adds 158 tokens to every session and 6,606 once invoked, about $0.0006 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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